Re: [PATCH v2 5/5] powerpc/lib: inline memcmp() for small constant sizes

Segher Boessenkool
2018-05-18T15:21:14Z

On Fri, May 18, 2018 at 12:35:48PM +0200, Christophe Leroy wrote:
> On 05/17/2018 03:55 PM, Segher Boessenkool wrote:
> >On Thu, May 17, 2018 at 12:49:58PM +0200, Christophe Leroy wrote:
> >>In my 8xx configuration, I get 208 calls to memcmp()
> >Could you show results with a more recent GCC? What version was this?
>
> It was with the latest GCC version I have available in my environment,
> that is GCC 5.4. Is that too old ?

Since GCC 7 the compiler knows how to do this, for powerpc; in GCC 8
it has improved still.

> It seems that version inlines memcmp() when length is 1. All other
> lengths call memcmp()

Yup.

> c000d018 <tstcmp4>:
> c000d018: 80 64 00 00 lwz r3,0(r4)
> c000d01c: 81 25 00 00 lwz r9,0(r5)
> c000d020: 7c 69 18 50 subf r3,r9,r3
> c000d024: 4e 80 00 20 blr

This is incorrect, it does not get the sign of the result correct.
Say when comparing 0xff 0xff 0xff 0xff to 0 0 0 0. This should return
positive, but it returns negative.

For Power9 GCC does

lwz 3,0(3)
lwz 9,0(4)
cmpld 7,3,9
setb 3,7

and for Power7/Power8,

lwz 9,0(3)
lwz 3,0(4)
subfc 3,3,9
popcntd 3,3
subfe 9,9,9
or 3,3,9

(and it gives up for earlier CPUs, there is no nice simple code sequence
as far as we know. Code size matters when generating inline code).

(Generating code for -m32 it is the same, just w instead of d in a few
places).

> c000d09c <tstcmp8>:
> c000d09c: 81 25 00 04 lwz r9,4(r5)
> c000d0a0: 80 64 00 04 lwz r3,4(r4)
> c000d0a4: 81 04 00 00 lwz r8,0(r4)
> c000d0a8: 81 45 00 00 lwz r10,0(r5)
> c000d0ac: 7c 69 18 10 subfc r3,r9,r3
> c000d0b0: 7d 2a 41 10 subfe r9,r10,r8
> c000d0b4: 7d 2a fe 70 srawi r10,r9,31
> c000d0b8: 7d 48 4b 79 or. r8,r10,r9
> c000d0bc: 4d a2 00 20 bclr+ 12,eq
> c000d0c0: 7d 23 4b 78 mr r3,r9
> c000d0c4: 4e 80 00 20 blr

> This shows that on PPC32, the 8 bytes comparison is not optimal, I will
> improve it.

It's not correct either (same problem as with length 4).

Segher